What are the different Opt-in Settings for mailing lists?

Let us begin by going over what Opt-in means in the digital marketing lingo:

Express permission by a customer, or a recipient of a mail, email, or other direct message to allow a marketer to send merchandise, information, or more messages.

A. The Opt-in forms

Opt-in has two common forms, Single Opt-in and Double Opt-in.

Single Opt-in:

An email address is added by any means to a mailing list but no process is followed in order to ensure that the address belongs to whomever submitted it, or that any form of formal permission was given by the owner. So, a single opt-in registration is when confirmed consent of an email's owner to receive messages is not requested.

Double Opt-in:

An email address is added by any means to a mailing list and a confirmation email is sent to verify that the person the email belongs to does give their consent to receive messages, usually by simply clicking on a link which is inserted for that purpose. On occasion, consent can also be given externally, even offline. So, a double opt-in registration is when confirmed consent of an email's owner to receive messages is requested.

B. Moosend list Opt-in Settings

A Moosend mailing list can have one of three different Opt-in Settings as shown below:

Option A : Use single Opt-in for this list

This setting is the default setting for lists. No consent is requested from the email address owners. When a valid email address is added to the list by any means, it is eligible to receive messages. Single opt-in subscriptions are best followed by welcome messages thus promptly giving each email address owner an easy way to unsubscribe from the mailing list, if they wish to.

Option B : Use double Opt-in for this list but continue sending to all subscribers

This setting enables a consent verification email to be sent out when an email address is added to the mailing list but does not exclude non verified subscribers from receiving messages. Please note that after General Data Protection Regulation (EUGDPR) becomes enforceable on May 25th 2018, this setting will not comply with the requirements for all unconfirmed subscribed individuals within the European Union.

Option C : Use double Opt-in for this list and only send to verified subscribers

This setting enables a consent verification email to be sent out when an email address is added to the mailing list and excludes all non verified subscribers from receiving messages. After GDPR (General Data Protection Regulation) becomes enforceable on May 25th 2018, this setting will comply with the requirements for all unconfirmed subscribed individuals within the European Union. Subscribers who have not verified their consent to receive messages will not be counted towards your monthly plan or be shown in your mailing lists and their totals.

With all above A, B & C, list opt-in options, account admins are able to manually verify a subscriber's consent, in case they have acquired it by other means. Account admins are however responsible for proving the subscriber's consent in the aforementioned scenario, should that be required by the subscriber for any reason.